I had to look it up too. Here's for those still wondering:
Jelqing is a penis enlargement exercise. It's basically done with a "milking" motion. Gripping the penis at the base and slowly moving toward the head.

Never heard of it till this thread either. Interesting it causes dysfunction because don't some trans* people do something similar to get bigger there? Would be interesting to know how it affects them too

I'm not a medical expert, so take my comments with a grain of salt (then again, most sites and individuals advocating jelqing aren't medical experts themselves, so there's that).

As of today, there have been no scientific studies showing any relationship between jelqing and an increase in penis size (mostly because few studies on the subject have been done at all). Furthermore, I've read that the "logic" behind jelqing--something like "stimulating the penis with certain pulling/milking motions stimulates the blood vessels and encourages growth"--is based on absolutely nothing; blood vessels just don't work or "grow" that way. And finally, yes, I've heard that jelqing can lead to ED; however, I'm somewhat skeptical about this claim as well considering the overall lack of studies on jelqing. I suppose the risks involve depend a lot on how rigorously you're jelqing. If we're talking basically something like a massage for your penis, then that's probably not going to be particularly harmful. But if you're grabbing your junk in a death grip and mashing/pulling it hard, yeah, that will definitely cause some problems.

If you're really interested in whether or not jelqing is working for you, don't just eyeball it; the fact that you WANT the technique to work involves a bias that'll make you more likely to "feel" like you're seeing a difference even if there's no real change over time. Instead, measure your erect length a few times per week and see if there's any significant change in the numbers over the next few months (only if you plan on continuing of course). And report back to us on your results (even if the result is "there's been no change"; that's interesting information too because it would tell other people not to waste their time with it).